Until recently, the prevailing belief was that lunar volcanism started soon after the moon formed, about 4.5 billion years ago, and ended about 3 billion years ago.

Scientists can determine the age of a lunar landscape by counting the craters that have been blasted into its surface by meteors. The older a region, the more craters it has.

There are fewer craters on the far side's lunar maria, or basalt seas, than expected, meaning they're younger than presumed.

The finding will lead the scientific community to reconsider the the early geology of the moon, said lead study author Jun'ichi Haruyama of the Japan Aerospace Exploration Agency, whose study appears in this week's issue of the journal Science.

moon exploration.

Ancient History

Scientists believe that early in the moon's formation—probably caused when a Mars-sized planet hit the Earth—light minerals floated to the top of a magma, or molten-rock, ocean, forming a harder crust.

Even after the crust had been fully formed, by about 3.2 billion years ago, the mantle melted occasionally and lava flowed on the lunar surface. Sometimes, meteor hits could trigger eruptions.

Most of the volcanism occurred on the near side in several phases, said Carle Pieters, a geologist at Brown University and a study co-author.

But there are relatively few basalts—glassy rocks formed by cooling magma—on the far side of the moon, so it was thought that volcanic activity had ended early in that hemisphere.